mdedetrich / cats-check

Cats instances for ScalaCheck data types




This is currently forked from here since it appears that it never got maintained (and it was never even published)


Cats-check associates ScalaCheck data types (i.e. Gen and Cogen) with Cats type classes (e.g. Monad).

Set up

Cats-check is not yet published.

Supported instances

The following instances are available for org.scalacheck.Gen:

  • MonadCombine[Gen]
  • Monoid[Gen[A]] (given Monoid[A])
  • Semigroup[Gen[A]] (given Semigroup[A])

The following instances are available for org.scalacheck.Cogen:

  • ContravariantCartesian[Cogen]
  • MonoidK[Cogen]


The easiest way to use these type class instances is with a single bulk import:

import org.scalacheck.instances.all._

You can just import all the instances for Gen (or Cogen) with the following imports:

import org.scalacheck.instances.gen._    // just Gen instances
import org.scalacheck.instances.cogen._  // just Cogen instances

You can also import instances by name if you prefer (although this approach is not recommended by the author):

import org.scalacheck.instances.gen.genMonadCombine

Custom Eq instances

In addition to the implicit instances (described above), this library provides explicit constructors you can use to build custom Eq instances. These instances cannot guarantee total equality, but can be used to rule out instances that can be shown to behave differently.

import cats.Eq
import org.scalacheck.Gen

// try to see if two gen instances are equal by comparing values
// generated for the same seed. If 20 iterations are successful
// then we consider the instances equal (though they might have
// an unseen difference).
implicit def genEq[A: Eq]: Eq[Gen[A]] =

// try to see if two cogen instances are equal by comparing seed
// permuations for the same 'A' value. if 20 iterations are
// successful then we consider the instances equal (though they
// might have an unseen difference).
implicit def cogenEq[A: Arbitrary]: Eq[Cogen[A]] =

Why would you want these?

Cats-check uses these instances internally to try to catch errors (i.e. law violations). During testing it's often useful to be able to provide these kind of "best effort" instances for types that wouldn't otherwise be directly comparable.

Frequently-asked questions

  1. Why use the org.scalacheck namespace?

    Some of the Gen methods this library needs to use are currently marked private[scalacheck]. Without these, implementing combineK, tailRecM, and sampledEq would be impossible (or at least very difficult). If ScalaCheck opens up these APIs, this may change.

  2. Where are the instances for Arbitrary?

    Since Arbitrary[A] just wraps Gen[A], there is little benefit to supporting instances directly. In the author's experience it's more likely that people will want to work with Gen[A] instances (obtained via arbitrary[A]) than Arbitrary[A] instances directly. (However, pull requests to add boilerplate-y Arbitrary instances woudl be accepted.)

  3. Does this library provide Arbitrary intances for Cats data types?

    It seems possible that cats-laws will provide these instances. If the Cats maintainers would prefer, we can definitely support adding these instances to cats-check.

Copyright and License

All code is available to you under the MIT license, available at and also in the COPYING file.

Copyright Erik Osheim, 2016.